What is the meaning of the designation ‘M30’ concrete suggested for a reinforced cement work?

What is the meaning of the designation ‘M30’ concrete suggested for a reinforced cement work?

a) The mix characteristic compressive strength of 150 mm size cube at 365 days is 30 N/mm²

b) The mix characteristic compressive strength of 100 mm size cube at 28 days is 30 N/mm²

c) The mix characteristic compressive strength of 150 mm size cube at 28 days is 30 N/mm²  (Ans)

d) The mix characteristic compressive strength of 150 mm size cube at 28 days is 300 N/mm²